A power supply device, such as a switched mode power supply or insulated inverter whose output exceeds 1 kW, is driven at about 10 kHz to 80 kHz from the viewpoint of efficiency. A typical example of the magnetic core material of a transformer for use in a switched mode power supply or the like which is driven in such a condition is a Mn—Zn ferrite. From the viewpoint of size reduction, a soft magnetic alloy material, such as an amorphous material or nanocrystalline material whose saturation magnetic flux density is high, can also be used. In a common configuration of the transformer, magnetic cores molded in a “UU” or “FE” shape are joined together in a coil form formed by winding a wire (conductive wire) around a bobbin beforehand, so as to form a magnetic path in a “” shape, racetrack shape, or “” shape.
In the above configuration, a gap occurs at the joint surfaces even though it is very small. Particularly when using a cut core formed from a soft magnetic alloy ribbon whose specific resistance is low, such a gap occurs so that a loss resulting from magnetic flux leakage increases. Thus, when the soft magnetic alloy ribbon is used in the form of a cut core, the operation magnetic flux density cannot be sufficiently increased, and it is difficult to say that a design which fully exploits the properties of the soft magnetic alloy material is possible.
Meanwhile, there is a transformer which uses an uncut core, such as a toroidal transformer. Here, an uncut core is sometimes referred to as “no-cut core” in comparison to “cut core”. However, winding of a wire in the toroidal transformer is manually carried out, and therefore, a problem of poor manipulation convenience arises. Further, it is difficult to make the state of the wound wire uniform, so that a problem of large property variations, etc., arises due to the effect of parasitic capacitance caused by the nonuniformly-wound wire. Patent Document 1 discloses, for example, the technique of efficiently winding a wire around an uncut magnetic core. Specifically, Patent Document 1 proposes a structure which is capable of mechanical winding by rotating a bobbin with the use of a driver. A reel (bobbin) disclosed in Patent Document 1 is shown in FIG. 18. In this bobbin, the circumferences of flanges 315 at opposite ends of a barrel 312 around which a coil is to be provided have teeth which are configured to mesh with driver teeth. The inner lateral surface of the flange 315 has a groove 318 in which an end portion of the wire at the start of winding is to be inserted and engaged for securing the wire to the flange 315. The groove 318 is provided for the purpose of preventing the starting end portion of the wire that is to form the coil from hindering rotation of the bobbin.
Patent Document 2 discloses a bobbin which has a different configuration. FIG. 19 shows an external view of the bobbin. This bobbin has restriction walls 415 which are provided on the inner side of flanges 414 that are provided at opposite ends of a barrel 425 and which have a smaller diameter than the flanges 414. The spaces between the flanges 414 and the restriction walls 415 are used as grooves 427 around which coil end portions are to be wound. An end portion of a coil (not shown) that is to be provided around the barrel 425 is wound around the groove 427. A wire which is to form the coil is passed to the barrel 425 through an insertion groove (not shown) provided in the restriction walls 415. Rotational force is applied to the flanges 414 such that a coil is evenly formed around the barrel 425. The flange 414 has a nail (not shown) on the groove 427 side such that the end portion of the coil would not be forced out of the groove 427.